The invention relates to a hydraulic drive apparatus and combination steering system for an industrial vehicle, in particular a four-wheel drive tractor as used in rough terrain.
Hydraulic drive systems for tractors have been used for many years, particularly four-wheel hydrostatic drive systems which have particular applications in terrain where there is poor traction, for example in agricultural, forestry, construction and mining applications. It is common for such vehicles to have hydraulic motors provided in the wheels, the motors being supplied with pressurized hydraulic fluid from a control system designed to reduce wheel slippage in poor traction conditions. It is known to provide the two motors of an oppositely disposed pair of wheels to receive pressurized hydraulic fluid from a variable displacement pump, the pump transmitting fluid to the wheel motors disposed in parallel. A flow divider is commonly fitted upstream of the wheel motors of the pair i.e. before motor inlets when driven forwardly, to ensure that the fluid flow is divided essentially equally between the wheels. In this way, should a wheel of the pair lose traction and slip, thus tending to rotate at a higher speed than the wheel maintaining traction, the flow divider ensures that the slipping wheel does not receive much more fluid than the wheel maintaining traction. Thus, power is still applied to the wheel maintaining traction and increases the chance of the vehicle being able to proceed. However, if the vehicle descends a slope in the forward drive mode, and the motors are used to brake the vehicle, if a flow divider is located upstream of inlets of the motor, low pressure can be generated at the inlet of at least one motor, and this can cause severe damage to the motor due to cavitation. Chances of cavitation occurring can be reduced by use of anti-cavitation valves.
When such a vehicle is operated in reverse, the flow divider is now located downstream from the wheel motors, and thus operates as a flow combiner and thus attempts, in a similar manner, to ensure that both wheels transmit essentially equal volume flows of fluid.
It is well known that use of a flow divider/combiner dissipates energy, and consequently heats the hydraulic fluid. If a vehicle is working continuously, and the flow divider/combiner is operating continuously, continuous pressure loss generates considerable heat which requires an adequate heat sink, typically an air-cooled hydraulic fluid or oil cooler. In some circumstances, the capacity of the cooler is insufficient and damage can result to the equipment from running on over-heated hydraulic fluid.
There are many patents relating to hydrostatic vehicle propulsions systems, typical patents being as follows: U.S. Pat. No. 3,900,075 (Chichester et al); U.S. Pat. No. 3,910,369 (Chichester et al); U.S. Pat. No. 3,916,625 (Holtkamp) U.S. Pat. No. 4,244,184 (Baldauf et al) and U.S. Pat. No. 5,199,525 (Schueler).
Also, when a hydrostatic drive vehicle negotiates a turn, to avoid wheel scuffing, wheels on the outside of the turn rotate faster than wheels on the inside of the turn, and thus the motor for the outside wheel requires a greater flow of fluid therethrough than the motor for the inside wheel. The difference in fluid flow rates between inside and outside wheels of a turn, hereinafter termed "flow differential", presents problems if a flow divider is provided in the inlet circuit, which attempts to maintain a constant flow through the wheels irrespective of the turn. Various devices have been devised to accommodate these problems and some of these devices are relatively complex and still result in scuffing of wheels during a tight turn.
For example, the above U.S. Pat. Nos. 3,900,075 and 3,910,369 disclose use of a proportional flow divider controlling flow to a pair of wheels, in which the flow divider proportions or distributes flow between the wheels in proportion to angle of the turn but, as discussed above, continuous use of a flow divider can generate excessive heat. Furthermore, accuracy of flow proportions of some proportional dividers/combiners is fairly poor, and thus the flow divider often can proportion flow improperly, causing excessive wheel scuffing during turning.
There are two main ways of steering such vehicles, namely modified Ackerman or kingpin steering, as used on most road vehicles, or articulated steering, which is commonly used in rough terrain vehicles, e.g. industrial tractors, which, in the logging industry, are termed "skidders". In modified Ackerman steering, the front wheels swivel about respective, generally vertical kingpins or equivalents which support generally horizontal stub axes which journal the wheels. When negotiating a turn, the front wheels follow an arc, and the back wheels tend to trail after the front wheels, on an inside of the turn and generally following a chord of the arc of the front wheels. Thus, when operating the vehicle in confined spaces, the rear wheels tend to trail the front wheels and require additional space on the inside of the turn.
The problem of trailing rear wheels is overcome by articulated steering, in which the vehicle has front and rear units connected together for articulated steering about a generally vertical articulation axis. Each unit has at least one pair of laterally spaced wheels, which are commonly mounted on fixed beam axles with respect to the units, so that the wheels can rotate about horizontal axes with respect to each unit, but do not rotate about vertical axes. Such vehicles are rugged and relatively successful in some applications, but have limitations in other applications on sensitive terrain. One problem is that, as the vehicle negotiates a turn, the front and rear units are articulated or inclined relative to each other at an angle, and the front and rear wheel pairs execute essentially identical arcs centred on a turn centre. In a normal turn, with no slippage, the rear wheels can follow essentially exactly in the paths of the front wheels, and this can damage delicate or sensitive terrain, particularly if the wheels start to slip due to poor traction. U.S. Pat. No. 3,414,072 (Hodges Jr. et al) and U.S. Pat. No. 3,910,369 (Chichester) disclose such vehicles.
In contrast, U.S. Pat. No. 4,042,053 (Sieren et al) discloses a four-wheel drive tractor with articulated steering between front and rear units, but with the front wheels also mounted for kingpin steering. This patent discloses a mechanical, as opposed to a hydraulic, tractor which has direct, mechanically powered wheels and steering units, and uses transmission shafts and gears as opposed to hydraulically powered motors etc. and which is therefor relatively complex. This patent shows a vehicle with a combination of front wheel kingpin steering and articulation steering which has several advantages. One of the advantages recognized by the patentee is that for a given radius of turn, the front wheels are inclined to the front unit and the rear unit is inclined to the front unit at shallower angles (i.e. less acute angles) than otherwise would be required for a normal kingpin steering or normal articulation steering vehicle. Use of shallower angles improves efficiency of power transfer to the wheels, as well as tractive effort for any load pulled by the tractor. In addition, because the units operate at shallower angles to each other than normal, any mechanical transmission components are operating in improved alignment with each other, which reduces wear and power consumption, thus improving life of the vehicle.